1. Field of the Invention
The present invention relates to electrically insulating prepregs which have excellent adhesive and thermal resistance properties, excellent mechanical strength and electrical insulating properties and especially good tracking resistance.
2. Description of the Prior Art
Generally, electrically insulating prepregs are used for the preparation of various shapes of laminated products such as laminated boards, laminated rods and laminated tubes by molding the prepreg or by preparing an electrical insulated conductor by coating or encapsulating the conductive material. The electrically insulating prepregs preferably are not very tacky at room temperature and are stable upon storage over long periods of time.
Among the necessary properties which the laminated products prepared from the electrically insulating prepregs must possess are satisfactory electrical and mechanical properties and good adhesive and thermal resistance. Because electrical instruments are now being used in high voltage applications in outdoor or indoor polluted environments, it is necessary that they maintain excellent characteristics under these conditions. The laminated products used in these electrical instruments are required to have arc and tracking resistance characteristics which prevent a decrease in the electrical insulating properties of the instruments. Such losses in electrically insulating properties can occur by the formation of conductive passages resulting from the deposition of carbon by current discharges or leaking currents on the surface of the laminated products. In other words, the properties of the laminated products prepared from the prepregs, of course, include the properties of the prepregs. However, laminated products derived from the conventional electrically insulating prepregs have not had satisfactory properties because the properties of the prepregs have not been satisfactory.
The known electrically insulating prepregs have been prepared from phenol resins, epoxy resins, unsaturated polyester resins, diphenyloxide resin and aniline, B-stage resins impregnated into solid substrates and the like. Coventional prepregs have good adhesive and mechanical properties. However, the laminated products and the molded products prepared from the conventional prepregs have disadvantageously low electrical properties especially, low arc resistance and tracking resistance. This results in products which do not possess long life with regard to their insulating properties as electrical insulators for electrical instruments for use in outdoor or in polluted environments. The laminated products prepared from the conventional melamine resin prepregs have good arc resistance and tracking resistance. However, they have the disadvantages of being brittle, low thermal resistance and cracks are easily formed on the surface. The laminated products prepared from the conventional silicon resin prepregs have good thermal resistance, arc resistance and tracking resistance. However, the silicon resin prepregs have low adhesive and low mechanical strengths.
In an approach to solve the problems associated with the prepregs, it was believed that prepregs could be prepared by impregnating a solid substrate such as a fibrous substrate, sheet-like substrate or a complex substrate wth a resin composition of an alicyclic epoxy resin which had no aromatic rings in the molecules and a curing agent. In order to confirm this hypothesis, studies have been conducted in which prepregs were prepared from solid substrates and various resin compositions which contained various curing agents and various commercial alicyclic epoxy resins, such as Chissonox 221 manufactured by Chisso Co., and Araldite CY-183 manufactured by Ciba-Geigy Co. However, with these resins it was very difficult obtaining a stable B-stage condition.
A need therefore, continues to exist for pregregs which can be formed from stable B-stage resins which have excellent storage stability, whereby the laminated or coated products prepared from the prepregs have satisfactory adhesive, thermal resistance, mechanical and electrical characteristics and especially, high arc resistance and tracking resistance.